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Now let /0 denote the characteristic function of the set {0}, and for
n ^ 1 let/n denote the characteristic function of the interval ]!/( + 1), 1/ri]
in R. For each C e R, the series /0(Q +/i(0 +    +/() + * * * converges to
the value at  of the characteristic function of the interval [0,1]. For each
n 2> 0, let Fn be the image of E under the orthogonal projector/n() = PFn
(15.10.6). From the preceding discussion and from (15.11.8(iv)) it follows
that E is the Hilbert sum of the Fn (n S> 0). Also, in fact, F0 = {0}, because
F0 is the kernel Qi the operator B (15.11.6), and by virtue of (15.12.6) the
hermitian form (x, y) i-> (B  x \ y) is nondegenerate, that is to say the relation
(B*x\x) = Q implies x = 0, and so F0 = {0}.

It follows from above that PFn(dom(AO) <= dom(JV) and that PFnN- x =
NPFn  x for all x e dom(N). On the other hand, the definition of the functions
/ for n ^ 1 shows that the function gn() = C~V(0 (with &i(0) = 0) is a
bounded regulated function on R and therefore universally measurable
(7.6.1). Consequently we may write PFn = Bgn(B\ whence JVPFn = NBgn(B).
But we have shown (15.12.6) that NB is a continuous operator defined on E,
and hence the restriction Nn of N to Fn n dom(N) is a continuous mapping of
this subspace into Fn. Since PFrt(dom(7\0) c= Fn n dom(JV) it follows that
Fn n dom(JV) is dense in Fn; also Nn is clearly a closed operator on Fn, hence
by (15.12.2) we conclude that Fn c dom(N). Also, since N*N = NN*,
we may interchange N and N* in the above discussion, which does not change
B; hence the restriction N'n of JV* to Fn is also a continuous operator on this
subspace, and is evidently equal to the adjoint N* of Nn. It is now clear that
Nn is normal, and the proof of (15.12.8) is complete.

(15.12.9)    Let T be a closed (not necessarily bounded) operator on E.
Generalizing (11.1), we say that  e C is a regular value of T if the operator
T  7 is a bijective linear mapping of dom(T) onto E, and if the inverse linear
mapping JRr(Q : E -> dom( T) is continuous.

(15.12.10)    For C e C to be a regular value of a closed operator T, it is sufficient
that T U should be an injective linear mapping of dom(T) onto a dense
subspace L 0/E, and that the inverse mapping (T  C/)""1 : L- E should be

For the graph of T C/ is closed, and so is the graph of (r C/)"1
(15.12.5). Hence it follows from (15.12.2) that L is closed in E, and there-
fore L = E.

The complement in C of the set of regular values of T is again called the
spectrum of T, and is denoted by Sp(T). If C e Sp(T), it follows from (15.12.10)
that there are three possibilities:ciple" is satisfied,